Homogenous compositions of polymers and crystalline solids or cross-linking agents and methods of making the same

ABSTRACT

Elastomeric polymers are combined with chemical agents in the presence of supercritical fluids such as supercritical carbon dioxide by swelling the polymer with the supercritical fluid, and impregnating the swollen polymer with the chemical agent. The process can be conducted at relatively low temperatures and pressures such that degradation of the polymer or the chemical agent is avoided. The chemical agent is preferably a curing agent which includes functional groups that interact with functional groups on the polymer to create an association therebetween. The process is enhanced by pre-dissolving the chemical agent in a solvent which does not solubilize the polymer to a great extent, but which is itself soluble in the supercritical fluid. In addition, during combining, mechanical mastication is performed to create a free flowing powder.

FIELD OF THE INVENTION

The invention generally relates to a method of mixing crystalline solidswith polymeric materials and, more particularly, to a methodology whichutilizes supercritical fluids, such as supercritical carbon dioxide, tocombine solid materials with polymeric materials in a homogenous blend.

BACKGROUND OF THE INVENTION

Polymer technology has employed supercritical carbon dioxide as analternative fluid medium to replace harmful organic solvents. The use ofsupercritical carbon dioxide in the synthesis of polymers is discussedin Cooper, A. I., J. Mater. Chem. 10:207 (2000); Ajzenberg et al., Chem.Eng. Technol. 23(10), 829 (2000), and U.S. Pat. Nos. 5,496,901 and5,618,894, which are herein incorporated by reference. These referencesdescribe solubilizing monomers, including fluoromonomers, in carbondioxide, and then polymerizing the solubilized monomers to form polymersof interest. Supercritical carbon dioxide has also been used to extractlow molecular weight components from polymer matrices as discussed inMcHugh et al. Supercritical Fluid Extraction, Butterworth-Heineman,1994. In addition, supercritical carbon dioxide has been used as ablowing agent for the production of polymer foams as discussed inUtracki et al. J. Polym. Sci. Part B-Polymer Physics 39(3), 342 (2001)and Cooper, ibid. Furthermore, supercritical carbon dioxide is used forpolymer processing. See, Kwag et al., Ind. Eng. Chem. Res. 40(14), 3048(2001) and Royer et al., J. Polym. Sci. Polym. Physics 38(23), 3168(2000). Coating applications require dissolution or suspension ofpolymer in solvent. Supercritical carbon dioxide has been used as asolubilizing and suspending media because of its benign nature andsolvent characteristics as a function of temperature and pressure in thesupercritical state. See, for example, U.S. Pat. Nos. 5,696,195,6,034,170, and 6,248,823, all of which are herein incorporated byreference. There is a high affinity of amorphous fluoropolymers forsupercritical carbon dioxide (see, Kazarian, J. Amer. Chem. Soc. 118(7),1729 (1996). This may be due to interactions between carbon dioxidemolecules in the supercritical phase and C═O and C—F bonds in thefluoropolymer.

Several other references are related to combinations of fluoropolymersand supercritical fluids, such as carbon dioxide. U.S. Pat. No.5,530,049 to Tuminello describes compositions of perflourinatedpolytetrafluoroethylene (PTFE) dissolved in supercritical carbondioxide. Japanese Patent Application 98233244 describes purification offluoropolymers by dissolving the fluoropolymer in a medium whichcontains a supercritical fluid. U.S. Pat. No. 6,034,170 to Dee describescompositions of supercritical carbon dioxide and fluoropolymers wherethe ratio of hydrogen atoms to fluorine atoms is controlled. U.S. Pat.No. 5,821,273 describes the use of supercritical carbon dioxide as afoaming agent for fluoropolymers. U.S. Pat. No. 5,863,612 to Desimonedescribes preparing fluoropolymers from a composition includingfluoromonomers in supercritical carbon dioxide. U.S. Pat. No. 5,645,894to Trankiem discloses coating a razor blade with PTFE using a dispersionof PTFE in supercritical fluid. Japanese Patent Application JP 91205307describes a fractionating process involving fluorochemicals andsupercritical or subcritical carbon dioxide. U.S. Pat. No. 5,696,195 toMcHugh describes the production of foams or webs using supercriticalsulfur hexafluoride and tetrafluoroethylene polymer.

Fluoropolymers have superior chemical and solvent resistancecharacteristics, and excellent thermal stability. Because of theseproperties, fluoropolymers have been increasingly used in the chemicaland semiconductor industry. However, processing of certainfluoropolymers, particularly PTFE and “modified” PTFE, can be difficult.For example, PTFE is not moldable or extrudable. Rather, PTFE componentsare typically cut or shaved from billets of material. PTFE is oftenprocessed by techniques that resemble those for ceramics. PTFE issintered at 370° C. before being formed by processes such as ramextrusion (see Scheirs, Modern Fluoropolymers, John Wiley & Sons, NewYork, 1997). Recently, fluoropolymer thermolysis (burning) and sinteringhas been identified as a potential source of halogenated organic acidsin the environment (see Ellis et al., Nature 412:321-324 (2001)). Thesehalo-acids, such as trifluoroacetic acid are persistent in theenvironment, as they have no known degradation process (see Boutonnet etal., Human and Ecological Risk Assessment 5:59-124 (1999)).

U.S. Pat. No. 5,567,769 to Noda describes the preparation of ahomogenous blend of a styrene and methyl methacrylate or 1,2 butadienewhich involves the uses of supercritical fluid. In the process, thepolymers, which are thermodynamically immiscible, are expanded through anarrow opening, and then fluid evaporation results in the deposition ofa homogenous material. A similar technology is described in U.S. Pat.No. 5,290,827 to Shine.

U.S. Pat. No. 5,548,004 to Green describes the production of a coatingpowder made from two different organic materials which are combined withsupercritical carbon dioxide, agitated in situ, and then discharged intoa vessel that is at lower pressure than the vessel in which theconstituents are combined. U.S. Pat. No. 5,766,522 to Daly describes theproduction of a powder coating wherein PTFE and/or a thermosettableresin developed in a continuous extruder using supercritical carbondioxide and a stream of precursor chemical. The process produces apowder coating. Similar technology is described in U.S. Pat. No.6,114,414 to Daly. U.S. Pat. Nos. 5,672,667 and 5,530,077 to Desimonedescribe multiphase mixtures which are prepared with a variety ofpolymers in supercritical carbon dioxide.

German Patent De 42023320 to Benken describes using a supercriticalfluid such as carbon dioxide or an alkane to carry an impregnatingmaterial into the pores of a substrate. For example, flavorants oraromas can be impregnated into food and cigarette products. U.S. Pat.No. 5,968,654 to Lee describes modifying a polymer substrate bycontacting it with a fluorinated compound distributed within asupercritical fluid.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a new method of combiningpolymeric materials with additives such as crystalline materials,crosslinking agents, etc. which is effective at low temperatures andpressures.

It is another object of the invention to provide a new method ofcombining a crosslinking agent with a fluoroelastomeric polymer.

It is another object of the invention to provide powderized, homogenouscompositions which include polymeric materials combined with and/orcoating additives such as crystalline materials, crosslinking agents,etc.

According to the invention, a supercritical fluid (SCF) is used toexpand the polymer and make it more amenable to impregnation by theadditive (chemical agent). Under mild temperature and pressureconditions in the presence of SCF, the additive becomes associated withthe polymer and, with agitation or mastication, a free flowing powder ofpolymer/chemical agent particles can be produced. If the chemical agentis not soluble in SCF, it is first dissolved in a solvent that willdissolve in SCF but which will not itself solubilize the polymer to anysignificant extent. Preferably, the chemical agent is solubilizedvirtually to the point of saturation in the solvent. Then, the solventwith solubilized chemical agent is combined in SCF with the polymer, andimpregnates the polymer with the chemical agent. The solvent and SCF areremoved during or after mixing to produce a free flowing powder ofpolymer/chemical agent particles (e.g. polymer coated chemical agentparticles).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing an exemplary pictorial representation of theprocess steps for a fluoropolymer+additive+solvent+supercritical CO₂mixture.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

This invention allows combining chemical agents such as crystallinesolids, crosslinkers, and the like, with polymer materials to formhomogenous blends. The invention has particular application to theformation of powderized homogenous blends which can be molded, extruded,or subjected to other processing operations to yield components thatinclude the chemical agent uniformly distributed within the polymermatrix or forming part of the polymer matrix by being associated withthe polymer (e.g., complexed with the polymer or bonded (e.g., ionic,hydrogen, covalent) to the polymer). The polymers of interest includemost fluoroelastomers, hydrocarbon elastomers, and other amorphouspolymers. Preferred polymers will contain one or more substituent groupsthat can form a chemical complex, such as a proton donor-proton acceptorcomplex, with the additive. Typical elastomers within the context ofthis invention will contain repeat groups such as fluorinated ornonfluorinated ethylene, flourinated or nonfluorinated propylene,fluorinated or nonfluorinated vinyl ether, cyanide groups, hydroxylgroups, amine groups, carbonyl groups, vinyl groups, ethers, esters,aromatics, etc. Preferably the elastomers will be capable of beingcrosslinked in later processing steps and are thermoset resins.Preferably, the chemical complex will typically exhibit an interactionstrength that is greater than a physical interaction such as nonpolardispersion interactions. The additives (chemical agents) which may beused in the practice of this invention are preferably crystallinesolids, and include but are not limited to, high temperature meltingsolids that contain substituent groups that can form chemical complexeswith the substituent groups in the elastomeric polymer. The crystallinesolid additives typically will contain functional groups such as amines,hydroxyls, carboxylic acids, carbonyls, ethers, esters, aromatics, etc.The chemical agents may be crosslinking agents, dyes, pigments, fillers,and tougheners.

For exemplary purposes, a polymer within the practice of this inventionmay be an amorphous polymer that is a terpolymer containing, on average,one cyanide group per 100 repeat groups. The chemical structure of thisexemplary polymer is set forth below:

These polymers are generally produced by emulsion polymerization methodsusing ammonium persulfate as an initiator, the terminal ends are thoughtto be —COOH, —OSO₃H or their metal salts.

For exemplary purposes, a chemical agent within the practice of thisinvention may be a crystalline solid additive that has several aminesand have the chemical structure set forth below (T_(melt)=180° C.):

where X can be a hydrogen, or an amine moiety (or other functionalgroup), wherein in this example, preferably at least two X moieties areamines (NH₂). As can be seen from these exemplary structures, complexesbetween the amines of the additive and the cyano group of the polymermay result.

Under the preferred milder conditions of the present blending process,the chemical agent is predominantly non-covalently associated with thepolymer cyano groups. Under curing conditions, which may involve highertemperature and/or the use of a catalyst, these complexes may react andform a cross-linked matrix resulting in a thermoset material. It shouldbe understood that the powderized blend produced by the present processis preferably capable of curing in subsequent processing steps utilizingcuring conditions, such as molding or extrusion, to form a thermosetarticle.

As will be recognized by those of skill in the art, the invention may bepracticed with a wide range of different polymers (including withoutlimitation fluorelastomeric polymers), and chemical additives. Forexample, as will be discussed in more detail below,poly(ethylene-co-vinyl acetate) (Evac-28), which contains 28 wt % vinylacetate along the polymer backbone can be impregnated with additive A,and will form crosslinks between polymer chains. The molecular weight ofthe polymeric materials which can be used within the practice of thisinvention may vary widely, such as, for example, from less than 0.1 kdto more than 10,000 kd. The experiments below demonstrate that thefunctional groups on the polymer materials can vary widely (e.g., cyanosversus acetates), and can be any of a variety of moieties. Likewise, thefunctional groups on the chemical agents which are combined with thepolymers can vary widely, and the functional groups on the polymers andchemical agents used in the practice of this invention will be chosen soas to form an association (e.g., complex or bond (covalent, ionic,hydrogen, etc.)) between the chemical agents and the polymers. Finally,the experiments performed demonstrate the robust character of theinvention in that degree of substitution on the polymer chain can varywidely from (e.g., from one moiety every one hundred repeats as influoropolymer A or 28 wt % as in the case of poly(ethylene-co-vinylacetate).

In the invention, a supercritical fluid (SCF) is used to expand thepolymer material to make it more amenable to impregnation by the solidadditive. The SCF is preferably supercritical carbon dioxide (CO₂).However, other SCFs might also be used in the practice of this inventionincluding supercritical alkanes (hydrocarbons), fluorocarbons, water,ammonia, noble gases, and sulfur hexafluoride (SF₆). The principalrequirement is that the SCF solvent exhibit some solubility in theelastomeric polymer causing the polymer material to swell.

The SCF assisted mixing process of this invention contemplates combiningthe polymer with the additive (chemical agent in the presence of SCFunder mild temperature and pressure conditions. For example, thepreferred operating temperature range for the SCF assisted mixingprocess is from sub-ambient temperature to about 60° C. The invention isnot limited to using temperatures below 60° C.; however, it is preferredthat temperatures be selected within the practice of this inventionwhich do not adversely affect the stability of the additive, thepolymer, or the solvent used to solubilize the additive (if any). Thepreferred operating pressure range for the SCF assisted process is 400to 2,600 psia. Low temperatures (i.e., below 200° C.) and low pressures(less than 10,000 psi, and in the examples discussed herein, less than3,000 psi) allow a number of materials to be handled withoutdegradation. The invention is not limited to using pressures below 2,600psia since the process will depend on the phase behavior of the additive(chemical agent), the solvent (if any), and the SCF. In someapplications, the technique may be employed at pressures as high as30,000 psi and a temperature low enough that the additive does not reactor does not liquify.

Ultimately, a goal of the present invention is to produce a homogenousblend, and preferably a free flowing powder, of polymer associated withchemical agent (e.g., polymer coated chemical agent particles). In somecases, the chemical agent may readily dissolve in the SCF. However, inmost cases, the chemical agent will not readily dissolve in the SCF, andthis requires the use of a solution of chemical agent (pre-dissolvedchemical agent), which solution is added to a chamber or other vesselfor combining the chemical agent with the polymer in the presence ofSCF. As will be shown below, it is possible to associate the chemicalagent with polymeric material effectively at low operation temperaturesand low pressures if the chemical agent is first pre-dissolved,preferably virtually to saturation, in a suitable solvent that does notdissolve the polymeric material to any great extent, but where thesolvent does dissolve in the SCF. The solvent plus chemical agentsolution could range widely in weight percentage solvent to chemicalagent (e.g., 1:9 to 9:1) The solvents of the crystalline solid additives(chemical agents) used in the practice of this invention include, butare not limited to, aprotic solvents such as acetone that easilydissolve the crystalline solid additive at room temperature. Solventsare preferred that are liquids at room temperature and that can dissolvelarge quantities of crystalline solid additive at room temperature orslightly elevated temperatures. The principal requirements of thesolvent are that the SCF solvent used to swell the polymeric materialwithin the practice of this invention also be able to dissolve thesolvent used to dissolve the crystalline solid material (chemicalagent), and that the solvent does not dissolve the polymer to any greatextent (e.g., less than approximately 1% by weight).

If the homogenous blend of polymer and chemical agent is to bepowderized, a variety of mechanical mastication techniques can be usedwithin the practice of this invention including without limitation ballmilling and brabender mixing. In one embodiment of the invention, ballmilling is utilized wherein the elastomeric polymer, such as theexemplary fluoropolymer (fluoropolymer A) set forth above, and thechemical agent, such as the exemplary amino substituted fluorinatedsolid additive (additive A) set forth above, are combined in a mixingchamber which includes a number of stainless steel balls. The purpose ofthe stainless steel balls function to transfer mechanical energy to thepolymeric material and chemical agent to convert them to a powder. Inthis process, the chemical agent is first dissolved in a suitablesolvent such as acetone. Preferably, the chemical agent is present inthe acetone solution at virtually saturation level. The acetone solutionand polymer are combined with the SCF solvent (preferably supercriticalcarbon dioxide) in the chamber at low pressure and temperature. Themixture is heated, pressurized, and agitated with the ball bearings.Then, the resulting material is cooled and de-pressurized to remove thesupercritical carbon dioxide. Removal can be achieved by spraying theblend of polymer and chemical agent or simply by depressurizing (eitherabruptly (greater than 10 psi/min) or slowly (equal or less than 10psi/min)) the vessel containing the blend of polymer and chemical agent.Acetone, or other suitable solvents, are preferably removed with the SCFsince they are dissolved therein. Alternatively, solvents might also beremoved by evaporation or other suitable technique. The resultingmaterial is a powder which includes polymer/chemical agent particles. Anexemplary process methodology is set forth below:

1. Add fluoropolymer (or other desired and suitable polymer) to themixing chamber.

2. Dissolve the crystalline solid curing agent (or other suitablechemical agent) in a suitable solvent at room temperature (if thechemical agent is not readily soluble in the SCF).

3. Add a curing agent-solvent solution to the mixing chamber (if thechemical agent dissolves in the SCF the chemical agent can be combinedwith the polymer without a solvent).

4. Add stainless steel balls (if ball milling is used-other masticationtechniques can be employed) to the mixing chamber for the mechanicalmastication process.

5. Add SCF solvent to the mixing chamber.

6. Heat and pressurize the chamber at approximately 4° C./min (or othersuitable rate) until the target temperature and pressure is reached(which will depend on the polymers and chemical agents which are chosento be combined).

7. Mix the materials for a fixed amount of time (long enough tothoroughly intermingle the chemical agent and polymer, and long enoughto allow the polymer to be expanded by the SCF—the mixing time will varydepending on the materials and conditions which are used).

8. Remove heat and slowly vent the mixing chamber to atmosphericconditions.

9. Open mixing chamber and recover the polymer-chemical agent blend (inthe case of the two exemplary materials above, the blend will be afluoropolymer-curing agent blend).

10. If desired, sift the blend to recover a powder of desired particlesize.

The invention is particularly useful in forming powderized blends ofthermosetting resins and chemical agents, where the theremosettingresins are capable of being crosslinked. These powderized blends can bemolded, extruded, or processed by a variety of means and can becomepermanently hard or rigid when heated above a curing temperature so thatchains become irreversibly joined or bonded together. Preferably, theirreversible bonding or crosslinking is achieved by heating thehomogenous blend prepared by this invention to temperatures higher thanthose used in the blending process.

FIG. 1 provides a pictorial representation of some of the above processsteps wherein an SCF-assisted ball mixing cycle is used in the presenceof supercritical CO₂. The filled circles show the heating/pressurizingpart of the process, and the open circles show the coolingdepressurizing part of the process. For the exemplary compounds notedabove, and for this exemplary process, mixing lasts one hour at 70° C.and 2,500 psig. As can be seen from FIG. 1, depressurizing is typicallymore rapid than pressurizing, while the relative increase in heat ismore rapid than the heat decrease during depressurizing. Mixing occursunder temperature and pressure conditions which are optimized for thepolymer and additive utilized.

A number of experiments were conducted using the fluoropolymer A andadditive A noted above which demonstrated the viability and utility ofthis invention. In a series of preliminary experiments directed toassessing the various variables used in the process, the fluoropolymer Aand additive A were combined at atmospheric pressure and a temperatureof 60° C. in the absence of carbon dioxide. Mastication with stainlesssteel balls changes the physical appearance of the fluoropolymer in thisinstance from that of a clumpy powder to that of an open porousmaterial, but very poor mixing or blending occurred between thefluoropolymer A and the additive A. In the preliminary experiments, theflouropolymer A was exposed to supercritical carbon dioxide in themixing chamber at 60° C. and at 2500 psig, in the presence or absence ofa small amount of acetone. These preliminary experiments also resultedin a significant change in the fluoropolymer A morphology from theuntreated material. Specifically, the supercritical carbon dioxidechanges the morphology of fluoropolymer A from a clumpy material to anopen porous material due to the SCF solvent dissolving in thefluoropolymer A, thus expanding and swelling fluoropolymer A. When thepressure was relieved, the SCF solvent rapidly vents from thefluoropolymer A making the fluoropolymer A an open morphology withincreased surface area. However, the presence of acetone in thecombination had very little effect on the morphology of fluropolymer A.Finally, in the preliminary experiments, flouropolymer A and additive Awere combined in the mixing chamber with a small amount of acetone andwere exposed to processing conditions of 60° C. and 2000 psig. However,nitrogen (N₂) replaced supercritical carbon dioxide in this experiment.It was determined that nitrogen does very little to change thefluoropolymer A morphology. This is due to nitrogen only dissolving to avery low extent in fluoropolymer A. Hence, the effect of nitrogen issimilar to hydrostatic pressure alone. These preliminary experimentsshow that if SCF solvent exhibits reasonable solubility in the polymermaterial, the final polymer morphology is more open and there is anincrease in surface area of the polymer. Effective mixing of thefluoropolymer A with the additive A did not occur in any of these cases.

Table 1 shows how the inventive process works effectively and yieldshomogenous powderized blends of polymer and chemical agent. In Table 1,supercritical carbon dioxide assisted mechanical mastication wasperformed using an approximately 50 wt % additive A (curingagent)+acetone (solvent for additive A) solution with supercriticalcarbon dioxide and fluoropolymer A added to the mixing chamber and beingsubjected to ball mixing at various temperatures and pressures. TABLE 1wt % wt % wt % wt % % acetone poly add. add. fines Tem. Pre. poly A add.A acetone CO₂ in A in A in A in after Ex. # (° C.) psig gms gms gms gmsBM BM BM poly A sift 1 60 2500 2.00 0.13 0.18 71.3 0.2 2.8 0.2  6.7 69 260 2500 2.02 0.19 0.27 81.7 0.3 2.5 0.2  9.4 80 3 60 2500 2.15 0.42 0.4781.9 0.6 2.6 0.5 19.5 97 4 50 2600 1.99 0.79 0.79 82.3 0.9 2.3 0.9 39.421From Table 1, it can be seen that for all four examples (Ex. #), the SCFball mixing is operated at approximately the same temperature (Tem.) andpressure (Pre.) and with supercritical CO₂. In all four cases, apredetermined amount of the additive A (add. A) is added to the mixingchamber from an approximately 50 wt % additive A plus acetone solution,and mastication is achieved with a ball mixer (BM).

Table 1 presents the percentage of fines after sifting. When thefluoropolymer A and additive A blend is recovered, it was very gentlysifted, and the relative amount of material passing through the siftingscreen was determined. This sifting offers a crude measure of thevariation in particle size in the powder produced.

Experimental example 1 in Table 1 shows that 6.7 wt % additiveA+fluoropolymer A is created with 69% fines. Experimental examples 2 and3 show that with increasing concentrations of additive A, 9.4 wt % and19.5 wt %, respectively, the amount of fines increased to 97%. Themorphology of the materials for examples 1-3 was entirely different fromthe starting fluoropolymer A material alone, even though the blendsconsist of 93.3, 91.6 and 81.5 wt % fluoropolymer A, respectively. Theblends in examples 1-3 are free flowing powders. That is, the exposureto supercritical carbon dioxide changed the morphology of thefluoropolymer and allowed incorporation of the additive A to produce apowderized, free-flowing, non-clumping material that is a blend of thetwo materials with particles of additive A associated with fluoropolymerA. The increase in percentage fines suggests that the solvent (i.e.acetone in this instance) helps disperse the additive A throughout themixture and makes it easier for the SCF to transport the additive A tothe sites in the fluoropolymer A that contain functional groups that canform a chemical complex with the additive A.

Experimental example 4 shows the results when an excess of additive A isadded to the ball mixer. As can be seen from Table 1, only a smallamount of fines (21%) are obtained when the final blend is gentlysifted. Also, the blend recovered from the mixing chamber showed somediscoloration that suggested that the additive A, which is purple incolor, resided on the outside of the surface of the fluoropolymer A,rather than being impregnated within (or coated by) fluoropolymer A.This result suggests that if a curing agent is used as the chemicalagent within the practice of this invention, it will have its maximumeffect on forming a fine, free flowing powdered blend if the amount ofthe curing agent is less than or close to (e.g., equal, slightlygreater, etc.) the amount needed to titrate all of the functional sitesin the polymer with which it is combined.

In practice, the blending/powdering process of this invention works mosteffectively if enough additive is used so that all of the repeat groupswith an active site are titrated with additive. However, the processalso works very effectively if the ratio of additive-to-active polymeris greater than 1.0, which means that more than one additive molecule isassociating (e.g., bonding, etc.) to the active site. Also, the processworks effectively if the ratio is a little less than 1.0. In theseinstances “effective” means that greater than 80 wt % of thefluoropolymer charged to the process is converted to the free flowingpowdered material. Ideally, the ratio of additive to active polymer sitewill be less than or about equal to 1.0:1.0 (i.e., in a range ofslightly less than to slightly more than 1.0:1.0 of additive to polymeractive site).

Table 2 shows the effect of mixing temperature and pressure on thesupercritical CO₂ assisted mechanical mastication process of thisinvention using an approximately 50 wt % additive A+acetone solution.TABLE 2 wt % wt % wt % wt % acetone poly. add. add. Temp Pres. Poly Aadd. A acetone CO₂ in A in A in A in Ex. # ° C. psig gms gms gms gms BMBM BM poly. A 1 34 2300 1.99 0.27 0.27 84.2 0.3 2.3 0.3 13.7 2 60 10702.01 0.22 0.22 40.2 0.5 4.9 0.5 10.7 3 60 1740 2.06 0.21 0.21 41.7 0.64.8 0.4 10.2Experimental example #1 in Table 2 was conducted at lower temperatureand was not transformed into a free flowing powder. The blend was alsodiscolored suggesting an uneven distribution of additive A influoropolymer A. Experimental example 2 in Table 2 shows that with lowerpressures, a similar discolored, non-free flowing powder results. Theoptimum temperature and pressure varies depending on the polymer andchemical agent to be combined. As can be seen from Experimental example3 in Table 2, even when the pressure is elevated slightly from that ofexperimental example 2, the pressure is still not enough to yield a freeflowing powderized material.

Table 3 shows the effect of supercritical CO₂ assisted mechanicalmastication on the blending of poly(butadiene) (PBD) with anapproximately 50 wt % curing agent plus acetone solution performed underconditions similar to the technique used with fluoropolymer A. TABLE 3wt % acetone wt % wt % Temp. Pres. PBD addi. acetone CO₂ in PBD add. AEx. # ° C. psig gms A. gms gms gms BM in BM in BM 1 60 2800 ˜2.0 0.2 082.9 0 2.4 0.2 2 60 2800 ˜2.0 0.2 0.21 82.9 0.2 2.4 0.2Polybutadiene does not include functional groups such as those whichappear in fluoropolymer A noted above. In addition, polybutadiene is nota fluorinated elastomer. When the same SCF-assisted mechanicalmastication process was performed using polybutadiene and additive Aunder conditions similar to those which were favorable when additive Aand fluoropolymer A were combined to create a free flowing powder (e.g.,60° C. and 2800 psig), the same results were not achieved. Inparticular, the recovered blend was not a free flowing powder. In thisinstance, polybutadiene does not have a cyano group to form a complexwith the additive A. Thus, in some applications, it may be necessary tohave a functional group in the backbone or side chain of the polymer(e.g., a fluoropolymer) that can form a chemical complex with theadditive material. When the ball mixing chamber is depressurized, thechemical complex would not let the chemical agent (additive) escape withthe vented CO₂.

Table 4 shows the effect of supercritical CO₂ assisted mechanicalmastication on the blending of poly(ethylene-co-vinyl acetate (EVAc-28)which contains 28 wt % vinyl acetate along the polymer backbone, with anapproximately 50 wt % curing agent (additive A) plus acetone solution(pre-dissolved) performed under conditions similar to the techniquedescribed above with fluoropolymer A. TABLE 4 wt % wt % wt % EVAc wt %add. % EVAc acetone 28 add. A in fines Tem. Pre. 28 add. A acetone CO₂in in A in EVAc- after Ex. # (° C.) psig gms gms gms gms BM BM BM 28sift 1 50 2600 .160 .015 .027 95.1 .028 .168 .015  8.3 23 2 50 2750 .209.072 .087 96.6 .090 .216 .074 25.5 48 3 50 2500 .197 .163 .176 97.8 .180.200 .166 45.4 43Table 4 demonstrates that the technique of the present invention wherebythe functionalized polymer is expanded in SCF and impregnated with achemical agent which interacts with the functional group, can beperformed with polymers other than fluoropolymer A (i.e., it isapplicable to non-fluoroelastomers), and can be performed with polymerswith a greater degree of substition than 1 per hundred monomeric units(e.g., the vinyl acetate content may range from 0.01 mol percent to 100mol percent), and, further, can be performed with functional groupsother than cyano moieties (e.g., acetates, etc.). It should also beunderstood that compositions including more than one polymer could becrosslinked or “cured” according to this invention so long as thechemical agent has functional groups that can interact with functionalgroups on the different polymers (e.g., two different fluoroelastomers,etc.). Likewise, it should also be understood that compositionsincluding more than one chemical agent could be prepared according tothis invention (for example, in pharmaceutical applications, both a drugand a colorant might be incorporated; in industrial componentapplications both a curing agent and a dye might be incorporated, etc.).In these instances, it may be advantageous to have at least one chemicalagent with functional groups that interact with those on the polymer orpolymers in the composition.

Fourier Transform Infrared (FTIR) and differential scanning calorimetry(DSC) analyses were performed to verify that the crystalline solidcuring agent (additive A) was present in the fluoropolymer A whensubjected to the SCF assisted ball milling operations under the optimumconditions set forth above. In addition, the analyses were performed toverify that the SCF assisted ball milled material has a greaterconcentration of additive A in fluoropolymer A than material blended byhand at room conditions (ambient temperature, pressure and humidity).For the blends of examples 1 and 2 from Table 1 above, a series of peaksbetween 2000 and 3000 cm⁻¹ were found in the FTIR spectra. The peaks inthis same 2000 to 3000 cm⁻¹ range do not appear in the spectra for purefluoropolymer A or pure additive A, so these peaks are likely due to astrong chemical interaction or complex between the cyanide influoroplymer A and the amine groups in additive A. A much smaller peakat 2400 cm⁻¹ was observed for the handmixed blend created at roomtemperature, which is also likely to the complex between the cyanidegroup and the amine groups. However, compared to the FTIR scans for theblends of examples 1 and 2, it can be concluded that a significantlygreater amount of complex formation results with the SCF assistedprocess. Comparison of DSC scans for the pure additive A, the purefluoropolymer A, and the blend of example 2 from Table 1, showed thepresence of the additive A peak when the blend is made. This alsosuggests that the additive is admixed with the fluoropolymer and that itis in crystalline form.

As another example of the SCF assisted (e.g., supercritical CO₂)blending of polymers and chemical agents, EVAc 28 was combined withbisphenol AF using the procedures detailed above. EVAc 28 has thechemical structure:

In the experiments, EVAc 28 had a molecular weight of approximately250,000. Of course, it should be understood that the molecular weight ofthe polymer employed can vary widely. EVAc 28 is commercially availablefrom Sigma-Aldrich and has no melting temperature. As discussed above inconnection with Table 4, EVAc 28 does not include a cyano functionalgroup like Fluoropolymer “A”. Rather, it includes a carbonyl.Furthermore, the carbonyl is positioned relatively close to the polymerbackbone. Bisphenol AF has the chemical structure:

Unlike Additive “A”, bisphenol AF lacks amine moieties; however,bisphenol AF includes hydroxyl moieties which can complex with thecarbonyl (acetate group) on EVAc 28. Bisphenol AF has a molecular weightof 336 and a melting temperature (Tm) of 160. Bisphenol AF iscommercially available from Fluorochem and other sources, and is typicalof curing agents used in many polymerization reactions. An important andpreferred feature of the chemical agent is that is a solid at roomtemperature, and has a melting temperature that is in excess of themelting temperature for the SCF assisted mastication process. In thisexample, bisphenol AF fits these criteria well.

Table 5 shows the effect of supercritical CO₂ assisted mechanicalmastication on the blending of poly(ethylene-co-vinyl acetate) (EVAc 28which contains 28 wt % vinyl acetate along the polymer backbone, with anapproximately 50 wt % curing agent (bisphenol AF) plus acetone solution(pre-dissolved) performed under conditions similar to the techniquedescribed above with fluoropolymer A+additive A. In this instance, theEVAc 28 pellets were pressed into a thin film and cut into small piecesbefore being added to the ball mixer. TABLE 5 wt % wt % wt % bsip. wt %EVAc bisp. AF % EVAc bisp. acetone 28 AF in fines Tem. Pre. 28 AFacetone CO₂ in in in EVAc after Ex. # (° C.) psig gms gms gms gms BM BMBM 28 sift 1 45 2600 .299 .160 .515 80.6 0.2 0.37 0.20 34.9 27 2 50 2500.262 .296 .192 76.4 0.2  .34  .38 53.1 67Table 5 demonstrates that the technique of the present invention,whereby the functionalized polymer is expanded in SCF and impregnatedwith a chemical agent which interacts with the functional group, can beperformed with a variety of different polymers (i.e., it is applicableto non-fluoroelastomers), and can be performed with polymers with agreater degree of substitution than one per hundred monomeric units,and, further, can be performed with functional groups other than cyanomoities (e.g., acetates, etc.), and still further, can be performed withadditives that do not contain amine functional groups (e.g., hydroxyls,etc.). From Table 5 it can be seen that the percent fines that areformed depends on the amount of additive relative to the amount offunctional groups in the polymer.

FTIR and DSC analyses were performed to verify the crystalline solidcuring agent bisphenol AF was present in the EVAc-28 when subjected tothe SCF assisted ball milling operations under the optimized conditionsset forth above. In addition, the analyses were performed to verify thatthe SCF assisted ball milled material has a greater concentration ofbisphenol AF in EVAC 28, than material blended by hand at roomconditions (ambient temperature, pressure and humidity). For the blendsof Examples 1 and 2 in Table 5 above, a series of peaks between 3700 and4000 cm⁻¹ range do not appear in the spectra of pure EVAc 28 or purebisphenol AF, so these peaks are likely due to a strong chemicalinteraction or complex between the acetate in EVAc 28 and the hydroxylgroups in bisphenol AF. Comparison of DSC scans for the bisphenol AF,the pure EVAc 28, and the blend of examples 1 and 2 of Table 5 showedthe presence of a bisphenol AF peak when the blend is made. This alsosuggests that the additive is admixed with the EVAc 28 and that it is incrystalline form.

In another example of the SCF-assisted mechanical mastication process ofthis invention being performed with fluoropolymers, a fluoropolymer withthe chemical structure:

(these polymers are generally produced by emulsion polymerizationmethods using ammonium persulfate as an initiator, the terminal ends arethought to be —COOH, —OSO₃H or their metal salts)was combined with the additive

Fluoropolymer “B” is commercially available from Daikin Industries ofJapan, and contains 65 mol % vinylidene fluoride, 14 mol %hexafluoropropylene, and 21 mol % tetrafluoroethlyene. Bisphenol AF(BAF) has two hydroxyl groups located at each end of the molecule. Table6 shows the effect of supercritical CO₂ assisted mechanical masticationon the blending of fluoropolymer “B” with an approximately 50 wt %curing agent (BAF) plus acetone (pre-dissolved) performed underconditions similar to that described above with fluoropolymer“A”+additive A. TABLE 6 wt % wt % wt % wt % % acetone fluoropolymer BAFBAF fines Tem. Pre. Fluoropolymer B BAF acetone CO₂ in B in in in afterEx. # (° C.) psig gms gms gms gms BM BM BM fluoropolymer B sift 1 602500 2.14 .211 .169 79.0 0.21 2.6 0.21  9.0 28 2 60 2500 2.13 .563 .48782.9 0.56 2.5 0.56 20.9 47Table 6 demonstrates that the present invention, whereby the polymer isexpanded in SCF and impregnated with chemical agent which interacts withthe functional group (an interaction of the functional group of theadditive with the functional group of the polymer (in this instanceCH₂CF₂), is applicable to semifluorinated elastomers and can beperformed with polymers with a greater degree of substitution than 1 perhundred monomeric units. Further, the invention can be performed withfunctional groups other than cyano moietes (e.g., vinylidene fluoride,etc.) and can be performed with additives that do not contain aminefunctional groups (e.g., hydroxyls, etc.). In Table 6, the percent finesthat are formed depends on the amount of additive relative to the amountof functional groups in the polymer.

The invention may be used to create polymers uniformly mixed withcrystalline solid additives including curing agents, colorants,pigments, toughening agents, etc., the blend of polymer and materialwould exhibit enhanced compatibility with other polymers of a similarchemical structure since the additive would be encased or associatedwith the polymer, and the polymer portion would exhibit a higher degreeof miscibility with the parent polymer material. The polymers caninclude biodegradable polymers used for drug delivery devices, and theadditive (chemical agent), in this instance, could be thetemperature-sensitive crystalline drug material. Polymer drug “devices”(i.e., pharmaceutical agents dispersed within a biodegradable polymer)can be effectively made in this instance since the operating temperaturecan be maintained below the condition that would degrade the drug.

While the invention has been described in terms of its preferredembodiments, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theappended claims. Accordingly, the present invention should not belimited to the embodiments as described above, but should furtherinclude all modifications and equivalents thereof within the spirit andscope of the description provided herein.

1. A process for producing homogenous blends of chemical agents andpolymers, comprising the steps of: combining, in the presence of asupercritical fluid, at least one polymer capable of expansion in thepresence of said supercritical fluid, which includes at least onefunctional group distributed at one or more locations along its chemicalbackbone, with at least one chemical agent, which includes at least onefunctional group which is capable of associating with said at least onefunctional group distributed at said one or more locations along thechemical backbone of said polymer, said combining step permits said atleast one chemical agent to impregnate within said at least one polymer;permitting said at least one functional group on said chemical agent toassociate with said one functional group distributed at said one or morelocations along the chemical backbone of said polymer, said permittingstep producing a blend wherein said at least one polymer is associatedwith said at least one chemical agent at at least one of said one ormore locations along the chemical backbone of said polymer; andseparating said supercritical fluid from said blend after saidpermitting step.
 2. The process of claim 1 wherein said at least onechemical agent includes at least two functional groups, each of whichcan associate with said at least one functional group distributed atsaid one or more locations along the chemical backbone of said polymerso as to either combine two polymer chains of said polymer or to linktwo locations of said one or more locations or one polymer chain of saidpolymer.
 3. The process of claim 2 wherein said two polymer chains ofsaid polymer are combined by said chemical agent.
 4. The process ofclaim 1 wherein said at least one polymer includes at least twodifferent polymers.
 5. The process of claim 1 wherein said at least onechemical agent includes at least two different chemical agents.
 6. Theprocess of claim 1 wherein said at least one polymer is selected fromthe group consisting of fluoroelastomers, hydrocarbon elastomers, andamorphous polymers.
 7. The process of claim 1 wherein said at least onefunctional group distributed at said one or more locations along thechemical backbone of said at least one polymer is selected from thegroup consisting of cyanide groups, hydroxyl groups, amine groups,carbonyl groups, vinyl groups, ethers, esters, and aromatics.
 8. Theprocess of claim 1 wherein said at least one functional group on saidchemical agent is selected from the group consisting of cyanide groups,hydroxyl groups, amine groups, carbonyl groups, vinyl groups, ethers,esters, and aromatics.
 9. The process of claim 1 wherein said at leastone polymer is poly(ethylene-co-vinyl acetate).
 10. The process of claim1 wherein said chemical agent is present in said combining step in anamount less than or about equal to an amount needed to titrate all ofsaid one or more functional groups distributed at said one or morelocations along the chemical backbone of said polymer.
 11. The processof claim 1 further comprising the step of agitating said polymer duringsaid permitting step.
 12. The process of claim 11 wherein said step ofagitating is performed by ball milling.
 13. The process of claim 11wherein said step of agitating is performed using a brabender apparatus.14. The process of claim 11 wherein at least one of said combining andsaid permitting steps are performed at temperature and pressureconditions which, absent said supercritical fluid and agitationoccurring said agitating step, are not sufficient to induce significantchemical reaction among said at least one polymer and said at least onechemical reagent.
 15. The process of claim 11 wherein at least one ofsaid combining and permitting steps are performed at a pressure of lessthan about 10,000 psi.
 16. The process of claim 1 wherein said removingstep is achieved during a spraying operation.
 17. The process of claim 1wherein said removing step is achieved by abrupt pressure reduction. 18.The process of claim 1 wherein said at least one polymer is abiodegradable polymer, and said at least one chemical agent is apharmaceutically active compound.
 19. The process of claim 18 whereinsaid at least one chemical agent further comprises a dye.
 20. Theprocess of claim 1 wherein said supercritical fluid is selected from thegroup consisting of supercritical carbon dioxide, supercriticalhydrocarbons, supercritical fluorocarbons, supercritical ammonia,supercritical water, supercritical sulfur hexafluoride, andsupercritical noble gases.
 21. The process of claim 1 wherein saidsupercritical fluid is supercritical carbon dioxide.
 22. The process ofclaim 1 wherein said at least one chemical agent is a crosslinkingadditive.
 23. The process of claim 1 wherein said at least one chemicalagent is selected from the group consisting of crosslinking agents,dyes, pigments, fillers, and tougheners.
 24. The process of claim 1further comprising the step of solubilizing said at least one chemicalagent in a solvent that dissolves in said supercritical fluid prior tosaid step of combining.
 25. The process of claim 24 wherein said step ofsolubilizing is performed prior to said step of combining.
 26. Theprocess of claim 24 wherein said at least one chemical agent issolubilized in said solvent to approximately to saturation.
 27. Theprocess of claim 24 wherein said solvent does not solubilize said atleast one polymer to a significant degree.
 28. The process of claim 11wherein at least one of said combining and permitting steps areperformed at a pressure of less than about 3,000 psi and a temperatureof less than about 200° C.
 29. The process of claim 1 wherein saidremoving step is achieved by slow pressure reduction.